An amount of communication data has been increasing due to the widespread use of the Internet. In order to cope with this, it is necessary to increase the capacity of a transmission path. As one of techniques for realizing a large capacity, there is digital coherent type optical communication. In this type optical communication, light intensity information and phase information are extracted from a received signal, and are demodulated by a digital signal processing circuit.
Here, in the digital coherent type optical communication, various processes are performed in a digital domain in order to remove noise. For example, Patent Document 1 discloses a technique in which, in a case where signal transmission is performed at a speed higher than a Nyquist rate, maximum likelihood sequence estimation is performed such that a symbol determination timing is detected in order to suppress interference between codes. Patent Document 2 discloses a technique in which, in a digital filter using Fourier transform and inverse Fourier transform, in order to suppress discontinuous points in the end of a frame of inverse Fourier transform output, when frames of Fourier transform input are generated, the frames are caused to be overlapped in a fixed section.